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SubjectRe: [PATCH 1/2] mm/gup.c: fix the wrong comments
On Mon, Apr 08, 2019 at 07:13:13AM -0700, Matthew Wilcox wrote:
> On Mon, Apr 08, 2019 at 10:37:45AM +0800, Huang Shijie wrote:
> > When CONFIG_HAVE_GENERIC_GUP is defined, the kernel will use its own
> > get_user_pages_fast().
> >
> > In the following scenario, we will may meet the bug in the DMA case:
> > .....................
> > get_user_pages_fast(start,,, pages);
> > ......
> > sg_alloc_table_from_pages(, pages, ...);
> > .....................
> >
> > The root cause is that sg_alloc_table_from_pages() requires the
> > page order to keep the same as it used in the user space, but
> > get_user_pages_fast() will mess it up.
>
> I don't understand how get_user_pages_fast() can return the pages in a
> different order in the array from the order they appear in userspace.
> Can you explain?
Please see the code in gup.c:

int get_user_pages_fast(unsigned long start, int nr_pages,
unsigned int gup_flags, struct page **pages)
{
.......
if (gup_fast_permitted(start, nr_pages)) {
local_irq_disable();
gup_pgd_range(addr, end, gup_flags, pages, &nr); // The @pages array maybe filled at the first time.
local_irq_enable();
ret = nr;
}
.......
if (nr < nr_pages) {
/* Try to get the remaining pages with get_user_pages */
start += nr << PAGE_SHIFT;
pages += nr; // The @pages is moved forward.

if (gup_flags & FOLL_LONGTERM) {
down_read(&current->mm->mmap_sem);
ret = __gup_longterm_locked(current, current->mm, // The @pages maybe filled at the second time
start, nr_pages - nr,
pages, NULL, gup_flags);
up_read(&current->mm->mmap_sem);
} else {
/*
* retain FAULT_FOLL_ALLOW_RETRY optimization if
* possible
*/
ret = get_user_pages_unlocked(start, nr_pages - nr, // The @pages maybe filled at the second time.
pages, gup_flags);
}
}


.....................

BTW, I do not know why we mess up the page order. It maybe used in some special case.

Thanks
Huang Shijie

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